MR elements in the form of MR heads without center-taps are presently being used in disk file systems and are planned for many future disk drives. These MR heads have typically been biased with a current, and either a current or voltage has been used as the signal to be amplified. In addition, amplifiers have been designed to also reduce current from flowing through an electrical short which may sporadically occur between the air bearing surface of the MR head and the disk. The disk (which is typically grounded) and the head (which is in general at some other potential) will complete a circuit to allow a significant current to flow when this short occurs, unless the amplifier is designed to minimize the magnitude of the short-circuit current. Voltage biasing, under ideal conditions, yields the distinct advantage over current biasing of constant current density in the magnetoresistive stripe, thereby reducing the risk of electromigration.
U.S. Pat. No. 4,786,993 discloses an amplifier circuit that voltage biases an MR element, but has high noise and provides no short circuit protection.
Commonly assigned co-pending application U.S. Ser. No. 07/517 093 filed May 1, 1990 (now U.S. Pat. No. 5,103,353, granted Apr. 7, 1992 discloses an amplifier that provides both short circuit protection and has low noise; but the MR element is current-biased and produces an output signal corresponding to dRh/Rh, where Rh is the resistance of the MR element and dRh is the magnetic-signal-induced change in resistance of the MR element. While this circuit operates satisfactorily, it does not provide constant current density in the stripe of the MR element.
U.S. Pat. Nos. 4,706,138 and 4,879,160 disclose other amplifier circuits which current bias the MR element and provide either short circuit protection or low noise, but not both.
There is a need for an amplifier circuit that (i) voltage biases the MR element, (ii) provides a significantly improved signal/noise ratio with lower resistance MR elements, and (iii) provides short circuit protection.